5′-Guanosine monophosphate (hereinafter referred to as “GMP”) is a food additive widely used as a flavor enhancer, like inosine monophosphate (hereinafter referred to as “IMP”). GMP elicits an umami taste and its use is dependent on monosodium glutamate (MSG) also being used. It is often used in synergy with IMP to increase the intensity of the umami taste of MSG.
Examples of the methods for the preparation of GMP known thus far include (1) the enzymatic degradation of yeast RNA, (2) direct microorganism fermentation to GMP, (3) microorganism fermentation to guanosine, followed by chemical phosphorylation, (4) microorganism fermentation to guanosine, followed by enzymatic phosphorylation, (5) microorganism fermentation to xanthosine 5′-monophosphate (hereinafter referred to as “XMP”), followed by conversion into GMP by a corynebacteria strain, and (6) microorganism fermentation to XMP, followed by conversion of XMP into GMP by Escherichia coli which has aminase activity. Of them, method (1) has difficulties of material supply and is economically non-beneficial and method (2) suffers from the disadvantage of being of low yield due to the membrane permeability of GMP. Thus, the other methods are widely used in industrial applications.
For the method in which XMP is produced and converted into GMP, it is critical to increase XMP productivity. For example, Korean Patent Application No. 10-1991-018016 discloses an XMP aminase-inactive strain capable of producing XMP in high yield, which is semi-auxotrophic for adenine and guanine, tolerant of guanosine analoges and very susceptible to lysozyme, an enzyme which destroying cell walls. Korean Patent Application No. 10-2001-000513 discloses a strain of Corynebacterium ammoniagenes which can directly accumulate XMP at high concentration in a culture medium and a method of producing XMP using the same. The strain is prepared by irradiating the mother microorganism with UV light, treating with the mutagen N-methyl-N′-nitro-N-nitrosoguanidine (NTG), and selecting a mutant tolerant of norvaline, an analog of valine which affects the biosynthesis of XMP. Korean Patent Application No. 10-2008-006537 describes a method of increasing XMP yield in which purN and purH, genes involved in the biosynthesis of XMP, are modified.
On the other hand, ATP productivity leads to XMP yield because ATP is involved in the biosynthesis of XMP. Also, The activity of malate dehydrogenase has a great influence on the production of ATP. However, nowhere are microorganisms and methods which are designed to enhance malate dehydrogenase activity for increasing XMP production yields mentioned in previous documents.
Keeping in the mind that it is important to increase the ATP productivity of the XMP-producing strain, the present inventors conducted intensive research and found a gene which is responsible for the increase. Also, it was found that a Corynebacteria strain transformed with a recombinant vector carrying two copies of the gene in tandem, could produce XMP in high yield.